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Volcanic carbon dioxide drove ancient global warming event

An extreme global warming event 56 million years ago was driven by massive carbon dioxide (CO2) emissions from volcanoes, during the formation of the North Atlantic Ocean, a study suggests.

Researchers from the University of Southampton in the UK used a combination of new geochemical measurements and novel global climate modelling to show that the Palaeocene-Eocene Thermal Maximum (PETM) was associated with a geologically rapid doubling of atmospheric CO2 in less than 25 thousand years - with volcanoes squarely to blame.

The PETM is the most rapid and extreme natural global warming event of the last 66 million years.

It lasted for around 150 thousand years and global temperatures increased by at least five degrees Celsius - a temperature increase comparable with projections of modern climate beyond the end of this century.

While it has long been suggested that the PETM event was caused by the injection of carbon into the ocean and atmosphere; the ultimate trigger, the source of this carbon, and the total amount released, have up to now all remained elusive.

It had been known that the PETM roughly coincided with the formation of massive 'flood basalts' - large stretches of ocean floor coated in lava, resulting from of a series of huge eruptions.

These occurred as Greenland first started separating from north-western Europe, thereby creating the North Atlantic Ocean, the vestiges of which are still continuing in miniature in Iceland today.

What has been missing is evidence linking these huge volcanic outpourings to the carbon release and warming that marks the PETM.

"In order to identify the source of carbon we first generated a new record of the change in ocean pH through the PETM, by measuring changes in the balance of isotopes of the element boron in ancient marine fossils called foraminifera," said Marcus Gutjahr, who led the study as a post-doctoral fellow at the University of Southampton.

Foraminifera are tiny marine plankton that live near the sea surface and the chemical makeup of their microscopic shells records the environmental conditions of the time when they lived, millions of years ago.

"Ocean pH tells us about the amount of carbon absorbed by ancient seawater, but we can get even more information by also considering changes in the isotopes of carbon, as these provide an indication of its source," said Professor Andy Ridgwell from University of California, Riverside in the US.

"When we force a numerical global climate model to take into account both sets of changes, the results point to the large-scale volcanism associated with the opening of the North Atlantic as the primary driver of the PETM," Ridgwell said.

The team found that the PETM was associated with a total input of more than 10,000 petagrammes of carbon from a predominantly volcanic source.

This is a vast amount of carbon ? some 30 times larger than all the fossil fuels burned to date and equivalent to all current conventional and unconventional fossil fuel reserves.

In their computer model simulations, it resulted in the concentration of atmospheric CO2 increasing from 800 parts per million (ppm) to above 2000 ppm.

The Earth?s mantle contains more than enough carbon to explain this dramatic rise and it would have been released as magma, pouring from volcanic rifts at the Earth's surface.